Bending a model with Simple Deform vs Lattice that's Simple Deformed
Different out-of the box outcome
My wild guess would be that you'll simply not get the same deformation. Propotional editing is using a smooth falloff by default, while the Simple Deform modifer is bending linearly...it's a nuance, but if you try, there is a different shape as outcome. Same for the lattice modifier. With the Simple deform modifier you basically get one thing, with the other you have more room, they are more flexible.
At the end I think he is suggesting to use the tool beacause without it you'll not get the exact deformation he was looking for.
You can easily experience the difference by doing the inverse: try to get a perfectly circular deformation using proportional editing. You'll find more challenging than you expect.


Another reason could be that you cannot straight use the Simple deform modifier, but you'll have to rotate your object from the XY plane to get a proper result, while in all the tutorial the shield is right on that plane.